Sophia here from Shopify. I'm happy to help you find out what this charge was for.
A bill for your Shopify subscription can only be issued once the 14-day trial ends, even if you've selected a plan beforehand. If you had started making sales before the trial ended, then charges may have come through for things like a domain purchase or shipping labels. But since you've mentioned you didn't use the trial much, it's more likely to have been app charges in this case.
Did you receive an email notification from Shopify about the invoice? Do you remember whether you followed these steps to close the store before the trial ended? If the charge came from Shopify, there should also be an 8-digit invoice number associated with it on your bank statement. Let me know if you see that number.
For security reasons, I will need to authenticate your account in order to discuss the charge details. I have sent you an email so that I can follow up with you privately once I've located this invoice. Whenever you have a chance, please respond to the email with your .myshopify.com URL to confirm which store we're looking into. Thank you so much!